Driver is not working as expected
That comes from the difference between degrees and radians. For some reason, the drivers evaluate rotation in radians no matter what the  Units are set to in the Scene settings.

To get the expression working you have to convert radians to degrees. So your expression would be.
360/Array * 0.0174532925 or 360/Array * pi/180 or even shorter pi*2/Array

There are 0.0174532925 radians in 1 degree, so just multiply your expression by the difference between degrees and radians.